As the packing material capable of measuring a monovalent cation and a divalent cation at the same time in cation chromatography, a weakly acidic cation exchanger produced by forming a film of poly(butadiene-maleic acid) (hereinafter, sometimes referred to as PBDMA) on a porous silica gel and then heating it at 180° C. for 4 hours is known (see, Chromatographia, Vol. 23, No. 7, pp. 465-472 (1987)).
However, the weakly acidic cation exchanger obtained by this method is insufficient in the adhesion between the silica gel support and a resin produced from the PBDMA film and has a problem in that the reproducibility is poor.
For solving this problem, a weakly acidic cation exchanger obtained by coating a weakly acidic cation exchange composition comprising a copolymer-crosslinked product of a vinyl compound with an unsaturated carboxylic acid and an unsaturated carboxylic acid anhydride-diene monomer copolymer has been proposed (see, JP-A-5-96184 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). This is a method where in forming a film on the surface of a spherical fine particle support, a compound having a vinyl group as the functional group is present together and a PBDMA copolymer-crosslinked product is produced at a high temperature, whereby the durability is enhanced.
According to the production method of a packing material reported in these two publications, a fine particle containing an organic material need be heated at a high temperature at the polymerization and therefore, these production methods are not suitable for use in industry.
JP-A-8-257419 proposes a packing material for cation chromatography, which is obtained by coating a cured product of a polyfunctional carboxylic acid compound and a polyfunctional epoxy compound on the surface of a porous support. In this publication, it is reported that a column improved in durability is obtained, however, since an ester bond is contained in the structure containing a functional group, problems such as deterioration due to hydrolysis are thought to occur.
Furthermore, improvement in the separation of monovalent cation, particularly sodium ion and ammonium ion, is not a purpose in all of these techniques proposed and the separation of these cations is insufficient.
Apart from these, a technique for improving the separation of sodium ion and ammonium ion has been reported (see, U.S. Pat. No. 5,968,363 and U.S. Pat. No. 5,875,994), where a crown ether or a functional group having a structure analogous thereto is introduced into a porous organic polymer particle to retard the elution of particularly potassium and ammonium and thereby improve the separation of sodium ion and ammonium ion. The purpose is satisfactorily achieved, however, this column is deficient in that the peak shape of particularly potassium is worsened and a solution containing an organic solvent need be used for the eluent.
Furthermore, a case of graft-polymerizing a copolymer consisting of maleic anhydride and ethyl vinyl ether to a polystyrene-base substrate is reported in Examples of these publications. Generally, according to a method using graft polymerization, a functional group may be surely introduced in a large amount, however, the polymer chain of copolymer introduced by this method mostly extends in the direction perpendicular to the base material surface and the film thickness increases in the portion containing a functional group on a support. Therefore, when the amount of a functional group introduced is increased, the packing pressure in column may increase or the separation efficiency of column may decrease.
As the column for use in a suppressor system, Ionpac CS16 (trade name) is commercially available from Dionex. In this column, a packing material for cation chromatography having a carboxyl group/a phosphonic acid group is packed and the separation of monovalent cation, particularly sodium ion and ammonium ion, is improved. However, this is a column for a suppressor system using an eluent having a concentration as high as 30 mmol methanesulfonic acid.
Under these circumstances, a weakly acidic cation exchanger which can realize high-level separation of monovalent cation in an appropriate eluent concentration condition and which can be used for the simultaneous analysis of monovalent and divalent cations, is being demanded.
In general, a packing material having a carboxyl group is used as the packing material for cation chromatography which is used for the simultaneously analysis of monovalent and divalent cations. The amount of the carboxyl group is considered to be suitably from 500 to 3,000 μeq/g, because if the amount is less than 500 μeq/g, satisfactory separation cannot be obtained, whereas if it exceeds 3,000 μeq/g, the elution of ion is excessively retarded. For obtaining good separation of ion, this may be attained by increasing the amount of the substituent introduced as much as possible, however, according to conventional methods for introducing a substituent, the increase in the amount of the substituent often affects the support shape, as a result, the separation is worsened.
As for the technique of reacting a polymer compound having a double bond with a maleic anhydride, an electro-phoretic coating method using a compound obtained by derivatizing a product resultant from a reaction of a butadiene copolymer and a maleic anhydride, and an aqueous dispersion solution for photographic coating solutions, obtained by reacting a compound resultant from a reaction of butadiene with a maleic anhydride have been reported (see, for example, JP-B-49-4059 (the term “JP-B” as used herein means an “examined Japanese patent publication”) and JP-B-60-21372). However, the production example of a weakly acidic cation exchanger is only that described in JP-A-8-257419.